Fibroblasts from Balb/C skin are not angiogenic when cultivated in vitro under standard conditions for 3 generations. Angiogenic capacity is acquired at the 4th passage but neoplastic transformation, defined by tumor formation after transplantation, appears only after the 11th passage. Epithelium from C57/Black mammary gland is not angiogenic; infection with Mu MTV induces angiogenic capacity but the cells are not neoplastically transformed until they are also treated with DMBA. Acquisition of angiogenic capacity in this system indicates increased risk of neoplastic transformation.